I made a dense forest on the edge of the map in the level editor and then used it to make a skybox. This skybox is a combination of the forest, the default sky textures, and clouds I generated with GNU Image Manipulator. Hope you like it!
The Textures:
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A probably excessive number of Screenshots
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Unfortunately, the skybox works best when you're very close to the ground. If you're high in the sky or close to the edge of the map, it loses its effectiveness at seeming like it's part of the map. Obviously, if the ground isn't the same texture it won't work very well on that map either. This was made primarily for a specific custom map I made, where you generally doesn't spend a lot of time up high or near the edges of the map.